Assisted suicide is outlawed in the UK, with the 1961 Suicide Act making it illegal to "aid, abet, counsel or procure the suicide of another". Helping somebody to die carries a prison sentence of up to 14 years.

But how that law is interpreted is unclear. The director of public prosecutions (DPP) has not prosecuted any relative of the 100 Britons who have gone abroad to end their lives at clinics run by the Swiss organisation Dignitas.

Purdy successfully appealed in June for a judicial review in the high court on the grounds that the DPP had acted illegally by not providing guidance.

Euthanasia or "mercy killing" has been "decriminalised" in a number of European countries, including the Netherlands and Belgium.

Several attempts to legalise assisted suicide in Britain have been rejected. The most recent, in 2006, was defeated in the House of Lords by 148 votes to 100.

A similar case to Purdy's arose in 2001, when Diane Pretty, a 43-year-old who suffered from motor neurone disease, sought to get immunity from prosecution for her husband if he helped her to die in the UK.

The DPP refused to give Pretty an undertaking not to prosecute her husband, which was upheld by the high court in August 2001 and the House of Lords in November that same year.

She said the UK law on assisted suicide infringed Pretty's human rights, under article two of the European convention – the right to life.

But the court upheld the House of Lords' ruling, stating that article two was also there to protect life and that an intentional consensual killing in the context of "voluntary euthanasia" was regarded in English law as murder.